A surgical instrument designed to accurately determine the depth of a recess (usually a drilled hole). It typically consists of a moveable (sliding) centrepiece (a prong), the tip of which is inserted into the recess; when the tip makes contact with the bottom a reading is visually taken from a scale marked on the side of the device. It is often used to measure the depth of holes being drilled in bone so that the surgeon can determine the length of the bone screw to be used. It is made of corrosive resistant materials [e.g., stainless steel, titanium (Ti)]. This is a reusable device intended to be sterilized prior to use.